1. Field
Systems and methods are disclosed for emitting electromagnetic (EM) energy.
2. Background Information
Surface plasmon polaritons are surface plasmons associated with incident light waves that result when free space electromagnetic waves couple to free electron oscillations (surface plasmons) in metal. Surface plasmon polaritons are lightwaves trapped on a conductive metal surface due to their interactions with electrons on the conductive metal surface.
Metal supports collective surface oscillations of free electrons. These collective surface oscillations can concentrate electromagnetic fields on the nanoscale, enhancing local field strength in a particular direction by several orders of magnitude. Plasmon characteristics can be accessed at optical and radio wavelengths. Normal propagating electromagnetic (EM) waves have constant phase and amplitude in the same plane. Surface plasmons and surface plasmon polaritons have planes of constant phase perpendicular to those of constant amplitude, i.e. both are forms of evanescent waves.
The primary responders to EM waves are electrons followed by polar molecules. Even low inertia electrons can fail to keep up with high frequencies depending on material used in constructing a detector. The dependence on material is described by the index of refraction (or dielectric constant or relative permittivity) and is a function of EM frequency.
This dependence on index of refraction and on frequency (or wavelength) is called a “dispersion relation”. Surface plasmons on a smooth planar metal display non-radiative electromagnetic modes; i.e., the surface plasmons cannot decay spontaneously into photons nor can light be coupled directly with surface plasmons.
The reason for this non-radiative nature of surface plasmons is that interaction between light and surface plasmons cannot simultaneously satisfy energy and momentum conservation; the conservation of parallel momentum is not satisfied as represented by the momentum wave-vector, k (where the magnitude of k=2π/λ, with λ being the EM wavelength). When surface plasmons and light are made to be in resonance, the result is a “surface plasmon polariton”. The surface plasmon polariton is an electromagnetic field in which both light and electron wave distributions match in their momentum vector, i.e. they have the same wavelength. This is also true for non-optical EM energy, e.g., radio waves.
Resonance and field enhancement can be made to take place if the electromagnetic momentum wave vector is increased as in a transparent medium with an index of refraction, n, to match incident EM energy to the surface plasmon momentum Wave-vector, or inversely, resonance can be achieved by roughening the metal surface to impose a surface impedance (i.e. along the dielectric/metal interface) in order to match free space electromagnetic waves to surface plasmons. In practice, momentum restrictions can be circumvented either by a prism coupling technique to shorten electromagnetic wavelength or by a metal surface grating, nano-structures such as holes, dimples, posts or statically rough surfaces. This resonance results in the fields associated with moving electron collections enhancing that of electromagnetic waves at the matched wavelength.
There are a number of known ways in which to create surface plasmon polaritons. For example, the Kretchmann-Rather attenuated total reflection configuration includes a dielectric prism mated with a thin metal film. For the Kretchmann-Rather configuration, a plasmon does not ride on the dielectric/metal interface. Plasmons arise instead on the back, metal/air interface. Once EM waves incident on the dielectric/metal interface exceed the so-called critical angle of total internal reflection they establish evanescent waves, which penetrate the metal film to some skin depth. Such light induced evanescent waves excite surface plasmon polaritons on the side of the metal opposite the dielectric.